The present invention relates to the field of optical radiometry. Optical radiometry is the science of measuring the surface temperatures of bodies by means of the optical radiation which they emit. The word "optical" refers to electromagnetic radiation covering the spectrum from gamma rays and X-rays through the ultraviolet, visible and infrared regions, ending at a wavelength of about 1 mm where radio wavelengths begin.
In connection with temperature measurements, the word "pyrometry" is often applied to that branch of radiometry which deals with hot or incandescent surfaces. "Optical pyrometry" (or "brightness pyrometry") makes use of visible light to measure incandescent body temperatures, while "radiation pyrometry" describes the same process but using infrared radiation. The term "radiometry" is broader than these, for it includes measurements of cold bodies.
Many optical pyrometers have been devised which measure visible light simultaneously in two wavelength regions. Use is made of the ratio of light intensities in order to overcome certain problems in the use of a single wavelength region. Such devices are called "two-color pyrometers" or "ratio pyrometers."
The ratio method may equally well be used with cooler objects, making use of two wavelength bands in the infrared region. In this case, the method is referred to as "two-wavelength radiometry" although the term "two-color radiometry" is often used. Other terms are also encountered which describe the above methods. Examples of these are "dual-wavelength," "two-band," "multi-spectral," "dichromatic" and "spectral radiance ratio" which are used with either "radiometry" or "pyrometry."
Ratio-radiometry has been in use for many decades as an extension of the basic radiometric method of temperature measurement. Its purpose is primarily to eliminate the effects of target surface emissivity by assuming that it is the same in both wavelength regions which are being sampled. Also, the method is able to compensate for any difference in emissivities, provided that the ratio of these is known and remains constant during the measurement.
The subjects of optical and two-wavelength radiometry have been described very fully in the prior literature, examples being the technical papers by Hornbeck, "Optical Methods of Temperature Measurement," Applied Optics, Volume 5, No. 2, February 1966, pages 179-186, and also by Horman, "Temperature Analysis from Multispectral Infrared Data," Applied Optics, Volume 15, No. 9, September 1976, pages 2099-2104. Therefore, these subjects are only briefly discussed herein.
The process of radiant emission from a theoretically perfect emitting surface (a blackbody) is described by Planck's Radiation Law, EQU J.sub..lambda. =c.sub.1 .lambda..sup.-5 (e.sup.c.sbsp.2.sup./.lambda.T -1).sup.-1 ( 1)
where
J.sub..lambda. =radiant intensity at wavelength .lambda. (watts/cm.sup.3) PA1 c.sub.1 =3.7.times.10.sup.-12 (watts.times.cm.sup.2) PA1 c.sub.2 =1.43 (cm.times.deg) PA1 .lambda.=wavelength (cm) PA1 e=2.718 (dimensionless) PA1 T=absolute temperature (deg. K).
For the temperature and wavelength ranges in which we will be interested, the exponential term in parentheses is sufficiently greater than unity that Equation (1) may be written: EQU J.sub..lambda. =c.sub.1 .lambda..sup.-5 /e.sup.c.sbsp.2.sup./.lambda.T ( 2)
For a non-blackbody surface, we introduce an emissivity value, E.sub..lambda., which reduces the emission by a given amount at each wavelength: EQU J.sub..lambda. =E.sub..lambda. c.sub.1 .lambda..sup.-5 /e.sup.c.sbsp.2.sup./.lambda.T ( 3)
Most often, the emissivity will vary with wavelength throughout the spectral region of interest. If it is relatively constant over some region, the surface is referred to as a greybody over that region. The emissivity of a surface may also vary with surface texture and with viewing angle, and frequently (as with metals) it will change as the surface temperature changes.
Assuming, however, that an emissivity E.sub.1 characterizes a surface over some wavelength band centered on wavelength .lambda..sub.1 and that E.sub.2 is the corresponding value at .lambda..sub.2, we may write for the radiant intensities in the respective bands: EQU J.sub.1 =E.sub.1 c.sub.1 .lambda..sub.1.sup.-5 /e.sup.c.sbsp.2.sup./.lambda..sbsp.1.sup.T
and EQU J.sub.2 =E.sub.2 c.sub.1 .lambda..sub.2.sup.-5 /e.sup.c.sbsp.2.sup./.lambda..sbsp.2.sup.T
of which the ratio can be reduced to: EQU J.sub.1 /J.sub.2 =(E.sub.1 E.sub.2)(.lambda..sub.2 /.lambda..sub.1).sup.5 e.sup.(c.sbsp.2.sup./T)(1/.lambda..sbsp.2.sup.-1/.lambda..sbsp.1.sup.)
The quantities .lambda..sub.1 and .lambda..sub.2 are known and are constant as is c.sub.2. We make the same assumption for E.sub.1 and E.sub.2 and can hence replace them by new constants for brevity: EQU J.sub.1 /J.sub.2 =Ae.sup.B/T
Taking logarithms of both sides, we have: EQU log.sub.e (J.sub.1 /J.sub.2)=(log A)+B/T
or EQU T=B/log.sub.e (J.sub.1 /AJ.sub.2) (4)
where EQU A=(E.sub.1 /E.sub.2)(.lambda..sub.2 /.lambda..sub.1).sup.5
and EQU B=c.sub.2 (1/.lambda..sub.2 -1/.lambda..sub.1)
Equation (4) is the "working equation" of ratio pyrometry, just as Equation (3) is for "monochromatic" pyrometry, the difference being that the latter contains E explicitly. However, the user should be aware that in the former case, although E.sub.1 and E.sub.2 are allowed to vary throughout the course of the measurements, their ratio must remain constant.
In principle, one has only to measure the respective radiant intensities in the two wavelength bands, over some defined part of the target surface, in order to be able to deduce the temperature at this region. If the wavelength bands are not widely separated in the spectrum, one can safely assume that E.sub.1 /E.sub.2 =1, unless one has prior knowledge to the contrary.
In practice, there are two basic ways of implementing the measurement, each with its advantages and disadvantages. Either a simultaneous measurement may be made by two detector/filter combinations, or a single detector may be used to view the surface sequentially through alternating filters.
In the simultaneous method, care must be taken to ensure that the detector responses are similar or that any differences are calibrated out. The method offers the advantages that there are no moving mechanical parts and that the response time of the system is limited by that of the basic detection system rather than by "chopping frequency" considerations in connection with the motions of the filters. Although individual detectors are frequently chopped in order to eliminate thermal drift problems, this can be done at higher frequencies than one can use in filter alternation.
The sequential method eliminates any uncertainties due to possible detector drift but may introduce questions of reliability if the rotating or oscillating filter system is not carefully designed and tested.
The second method is often used in mass-produced two-color pyrometers for use by semi-skilled personnel, where the design cost is easily amortized and where periodic calibration is not feasible.
For laboratory uses of band-ratio radiometry, the method is best implemented by use of a two-detector system, along with appropriate calibration procedures, and this is the approach taken in the present invention.